1. Technical Field
The disclosure relates to methods for manufacturing thermal pastes, especially for flip chip microelectronic packages. In particular, the disclosure relates to a vacuum extrusion method of manufacturing a thermal paste, in which the method include feeding the thermal paste into a chamber of an extruder, mixing the thermal paste at elevated temperatures, de-airing the thermal paste, and extruding the thermal paste out of the chamber through a die as a pre-form or into a cartridge, such that air channels and pseudo-grain boundaries are prevented from forming in the thermal paste. Such prevention results in the superior performance and reliability of the thermal paste during thermal/mechanical stressing.
2. Discussion of the Background
Thermal interface materials (TIMs) are extensively used in microelectronic packaging technology, such as flip chip packaging. In particular, TIMs facilitate the heat transport from the chip to the environment and accommodate the stresses that arise between the silicon die and heat spreader and/or cooling unit, as a consequence of the temperature differential between the parts and the different thermal expansion coefficients of the materials involved.
TIMs are typically highly filled composite materials consisting of metal or metal oxide particles, having a diameter of about 0.1 to about 100 microns, and an organic binder. One common type of TIM is a thermal paste. Pastes have the advantage of achieving intimate contact between the TIM and confining interfaces, and enable small chip to cooling unit gaps called the “bondline.” The smaller the bondline, i.e., the TIM thickness, the lower the thermal resistance, since in general the thermal conductivity of the TIM is the lowest of all materials involved. However, with decreasing bondline thickness, it is difficult for the paste to survive unchanged with the large temperature changes arising during chip operation and CPU power on/off. These temperature changes lead to thermal expansion/contraction of the confining interfaces (i.e. the die and spreader/hat) which translate into mechanical stresses. A good paste is expected to facilitate the thermal dimensional changes of all rigid parts in the cooling stack without degradation of its thermal performance.
Methods have been developed to analyze the behavior of thermal pastes during thermal/mechanical stressing (see Feger et al., IBM Journal of Research and Development, vol. 49, p. 699 (2005)), and several degradation mechanisms have been characterized, including growth and coalescence of entrapped air in the paste, paste detachment from one or both of the confining interfaces, bulk paste slip off the die, and air channel development in the paste. All degradation mechanisms significantly impact the efficiency of the paste in conducting heat away from the chip, but air expansion with coalescence and air channel development are the most severe and most common failure modes observed in the field.
Recently, it has been shown that air channels originate from the displacement of the liquid binder in channels that arise along “pseudo-grain boundaries” in the paste. “Pseudo-grain boundaries” are poorly mixed areas in the paste and can arise, as can air entrapments, either during the paste formulation process due to insufficient mixing or during the dispense and assembly process.
Current mixing protocols for paste formulation are batch processes in which distributive mixing and high-shear processing (for deglomeration) are separated into several steps. Such batch processes afford many opportunities for air entrapment and pseudo-grain boundary formation. During the production of paste pre-forms, as generally described in U.S. Pat. No. 6,444,496, pseudo-grain boundaries and air can be introduced. Further, air and pseudo-grain boundaries can be introduced if the paste is dispensed from a cartridge over the chip area in patterns that can lead to air inclusions and/or grain boundary formation.
In view of the foregoing, there remains a need for a method of manufacturing thermal pastes and their pre-forms which eliminates air inclusions and pseudo-grain boundaries therein, which results in TIMs with superior performance and reliability during thermal/mechanical stressing.